1. Field of the Invention
A reflective mode, magneto-optic spatial light modulator (MOSLM) device has at least one planar electrical conductor which functions both as an optical mirror surface and as the address conductor for random access, selective switching of an associated individual pixel element which is formed from a magneto-optic material that exhibits magnetic domain characteristics.
2. Description of Related Art
In the field of magneto-optic devices, materials that exhibit magnetic domain characteristics, the behavior of such magnetic domains, and the use of these magnetic domain materials in sensing and display devices have been described: see U.S. Pat. No. 4,563,236; U.S. Pat. No. 4,578,321; and, U.S. Pat. No. 4,550,983 all assigned to the same assignee as this invention.
Magneto-optic display devices can employ the Faraday effect which depends on the change in rotation of the plane of polarization of electromagnetic radiation, such as polarized light, as it is transmitted through a magnetic domain. In accordance with the Faraday effect, if the transmitted radiation is plane polarized, passed through a magnetic domain, and then through an analyzer, the intensity of the radiation which has been so transmitted is either greater than or less than that radiation which is transmitted through adjoining domains. This resulting contrast in radiation intensities is the basis for magneto-optic display devices.
The MOSLM.TM. device, which functions like a magneto-optic device, has become very popular as an electrically addressed spatial light modulator (SLM) for optical correlation and pattern recognition. (Ross, W. E. and Lambeth, D. N., "Advanced Magneto-optic Spatial Light Modulator Device Development", SPIE Proc., Vol. 1562, pp. 92 to 102, 1991.) The MOSLM device depends upon the use of a LIGHT MOD.TM. modulator (a product of Litton Systems, Inc.) The acronym LIGHT MOD identifies this modulator as a Litton Iron Garnet H (Magnetically) Triggered Magneto Optical Device which differentiates it from thermally triggered devices such as the magnetooptical modulator of U.S. Pat. No. 4,578,651.
The LIGHT MOD SLM like other magneto-optic devices, is electrically addressable and changes or switches the direction of magnetization M of a magnetic material formed into individual pixel or post elements through electrical conductors or drive lines that establish a magnetic field having an opposite direction of magnetization. (Hill, B. And Schmidt, K. P., "Fast Switchable Magneto-Optic Memory Display Components", Phillips Journal of Research, Volume 33, Nos. 5/6, 1978, page 211; and U.S. Pat. No. 4,114,191). An external bias field may be required (U.S. Pat. No. 4,114,191) or it may be omitted (United Kingdom Patent No. 1,180,334). When a magnetic field is established having a strength equal to or greater than Hk-4.pi. Ms, the reversal of the direction of magnetization or switching occurs.
This process of switching involves two parts: nucleation of a domain wall and the movement or propagation of the domain wall toward or to complete saturation. A magnetic domain wall is established by nucleation at a nucleation site within the magnetic material of the selected post element. This domain wall is formed between the initial condition of magnetization and the nucleated opposite magnetic condition. The remainder of the element is switched by propagating the domain wall through the thickness of the pixel element so that part or all of the entire element exhibits a direction of magnetization opposite to the initial condition, i.e., partial or complete saturation.
It is known that the magnetic field required for nucleation is greater than that required for propagating the domain wall because wall motion is limited by demagnetizing and coercivity effects. The field established by the selected row-and-column conductors diminishes rapidly as the distance from the conductors increases. Thus, it is understood that the value or strength of the magnetic field adjacent to the energized conductors is large while the field in the region of the post element farthest from the energized conductors is relatively small.
The MOSLM device like the LIGHT MOD SLM is made from a magneto optic film which is a high bismuth content, iron garnet film having a large lattice constant (a product of Litton Systems, Inc.) The bismuth substituted, transparent iron garnet film is grown on a non magnetic substrate, and has its direction of uniaxial anisotropy oriented perpendicular to the plane of the film of magnitude greater than the saturation magnetization of the film. The high bismuth content of this film enhances its optical characteristics and results in a softer garnet which facilitates ion mill structuring using a photo resist process. The film is structured into an array of pixel elements by etching completely through the magnetic film to a non-magnetic substrate. Each pixel or element can be made to function as a valve to the passage of electromagnetic radiation, such as polarized light, by changing or switching the magnetization state of the selected pixel. This state change is initiated by the action of combined magnetic fields through coincident current select of paired electrical conductors at a selected pixel element.
The LIGHT-MOD SLM and MOSLM device are acceptable magneto-optic lens; however, there is a continuing requirement in applied research and prototype developments, leading to production units, for improvements to be made; particularly for improved lens to be used in a reflective rather than transmissive mode.